In the prior art various methods have been used to form metal free dental restorations from glass-ceramic materials. Glass-ceramic materials because of their strength, translucency, non-toxicity and other physical properties are ideal materials for use in forming dental restorations. Because of their suitability, glass-ceramic materials have been used to form dental restorations for at least eighty years. In the prior art the most widely used means for forming dental restorations from mixtures of glass and ceramic materials is a process which utilizes a slurry of glass and ceramic particles. In this process a die is formed which is an exact replica of the remaining portion of the tooth to which the restoration is to be secured. It is understood by one skilled in the art that the remaining portion of the tooth has been prepared by the dentist in such a manner that the attachment of the restoration is facilitated. To start the process the dentist takes an impression of the prepared tooth to create a negative impression of the prepared tooth or teeth. This negative impression is then packed with a material to form a positive impression of the prepared tooth or teeth. This positive impression is called a die. Platinum foil is then pressed over the die to from a matrix which is essentially a foundation on which the dental restoration is built. In order to form up the dental restoration many layers of a slurry of a particulate glass-ceramic material are applied to the platinum foil matrix. As multiple layers of the slurry are built up and dried a semi-solid structure is formed which can be carved into the shape of the desired dental restoration. Once the desired shape is achieved the structure is then removed from the die . At this stage the structure is referred to as a green structure. The structure is then fired, during the firing process the particulate glass-ceramic material fuses into a solid mass. Because the green structure is formed from multiple layers of the dried glass-ceramic material uneven fusing may result. As a result of this uneven fusing the physical properties of the finished restoration may be detrimentally affected. As a result an inferior dental restoration results. Further, as can be seen from the above description, the overall process is very labor intensive.
Other methods for forming the green restoration have been considered in the prior art, for example, in U.S. Pat. No. 2,196,258, a mixture of particulate glass and ceramic material which incorporates a binder is packed into a flexible mold to form a green structure which is then fired to form a finished structure. Again, because the process entails the fusing of particulate material, uneven fusing may result and hence a weakened and inferior dental restoration may result.
To overcome the problems as described above the trade has recognized that in order to produce strong, translucent metal free dental restorations it would be desirable to form these restorations directly from a homogenous molten glass-ceramic material. It was realized that it may be possible to produce a satisfactory restoration by forcing a molten or plastic glass-ceramic material into a mold having a cavity in the form of the desired dental restoration. The prior art further recognized that the glass-ceramic material could be introduced into the cavity when the glass-ceramic material was in the liquid or in the plastic state.
A constant goal of the prior art as described, was to effect the molding process in a quick and efficient manner and in a manner that produces a dental restoration that has excellent definition and fit. In dental restorations definition is extremely important, as in order to have a satisfactory restoration the finest details of the original tooth must be reproduced. For example, for a dental restoration to be successful the margins must be sharp and well defined. It is in this area that the prior art molding processes are deficient in that it was not possible to achieve the desired degree of definition.
Further, it is desirable to produce a dental restoration in a short period of time in order to efficiently utilize the overhead of the dental laboratory and in order to minimize the labor content of the dental restoration.
Dental laboratories are not typically well funded operations. Therefore, in order to keep overhead to a minimum it is highly desirable that a suitable process for forming dental restorations utilize equipment which is relatively inexpensive. While the above described process fits this requirement the below described DICOR process does not.
As is discussed above, there are several prior art processes for the manufacture of dental restorations from glass-ceramic materials. A recent addition to the prior art is the DICOR process as sold by the Densply Corporation of York, Pa. In this process a dental restoration is formed by centrifugal casting of a molten glass-ceramic material. This process is further described in U.S. Pat. No. 4,431,420 issued Feb. 14, 1984 and related patents. Centrifugal casting has been extensively used in the casting of metals principally by the lost wax process. Further, this process has been imminently successful for hundreds of years for use in conjunction with metals, this success results from the fact that molten metals have very low viscosity and high density in the molten state, hence, they function very well in centrifugal casting processes. That is, because molten metals have a high density and a very low viscosity in the molten state centrifugal force is adequate for purposes of injecting the molten metal into a preformed mold cavity. In an attempt to produce dental restorations which have high definition the above mentioned DICOR process uses centrifugal force to form the desired dental restorations from a molten glass-ceramic material. Molten glass-ceramic materials have a much higher viscosity and a much lower density when compared to molten metals. For this reason, it is not possible to consistently drive a molten glass-ceramic material, by centrifugal force alone into a mold in order to produce a satisfactory dental restoration. That is, a molten glass-ceramic material cannot be driven by centrifugal force into a mold cavity with sufficient force in order to always get the required definition, necessary to form a satisfactory dental restoration. It is well recognized by one skilled in the art that in order to have a satisfactory dental restoration, excellent definition must be achieved in order to recreate the desired margins which are needed for the proper fit of a dental restoration into the human mouth.
Further, the DICOR process, is deficient as to the coloration of the glass-ceramic material utilized. The resulting Dicor dental restoration had an undesirable white color and must be glazed in order to produce satisfactory human coloration. As a result the coloration is only on the surface of the dental restoration, If adjustment by grinding is needed in the final installation of the restoration into the human mouth the glazing is removed thereby exposing the whitish base which contrasts with the glaze. This contrast is very unsatisfactory from an esthetic point of view.
In contrast to this deficiency, the restoration of the subject process is adapted to utilize glass-ceramic materials wherein the coloration of the resulting dental restoration, throughout, approximates human tooth coloration. Hence, if grinding is necessary in final fitting contrast between the surface of the dental restoration and the underlying base is not observed.
Undesirable contrast can also result from normal wear, where as a result of the grinding action of one tooth against another the glaze is worn away. Again, this is not a problem in this invention as the preferred glass-ceramic material has a uniform natural coloration throughout. It should be noted that the restoration of the subject invention may be glazed to achieve the exact shade desired.
In contrast to the above discussed prior art processes, the process of the subject invention utilizes a positive mechanically applied force for purposes of injecting the molten dental glass-ceramic material into the preformed mold cavity.
For a dental glass-ceramic material to be satisfactory for use in the formation of dental restorations the material should incorporate many or all of the following properties:
1. It must be inert and non-toxic in an oral environment. PA1 2. It must have sufficient structural integrity to resist the forces of mastication and generally must have a 3-point MOR of at least 30,000 PSI. PA1 3. Should be capable of being formed into forms which are compatible with the human anatomy using simple equipment. PA1 4. Should have esthetic qualities (coloration similar to human teeth with a slightly translucent appearance) which are compatible with human teeth and hence should be monolithic or glazable. PA1 5. Further, the glass-ceramic material must not absorb moisture or stain and it must be stress corrosion resistant. PA1 6. Likewise the glass-ceramic material should have wear characteristics which are similar to natural human teeth and should be compatible with other dental materials. PA1 7. The glass-ceramic likewise must have dimensional stability and resist thermal shock during processing and in particular it must have dimensional stability during subsequent heat treating processes wherein recrystalization is effected. PA1 8. Further, the glass-ceramic material should be compatible from a thermal expansion point of view with metals, stains, glazes etc. as are conventionally used to form dental restorations. PA1 9. In order to create an esthetically pleasing dental restoration it is necessary to glaze and stain the final dental restoration to the exact shade desired. In order to effect this glazing straining etc. the dental restoration must be heated to a temperature of about 950.degree. C. for each staining or glazing operation. Therefore, a satisfactory glass-ceramic material must be capable of withstanding multiple heat cycles to about 950.degree. C. PA1 10. It must be capable of retaining its structural integrity during heat treating. PA1 11. In summary a suitable glass-ceramic material should have; PA1 A. Thermal expansions of 130.degree. to 150.degree..times.10 /.degree. C. PA1 B. Translucency of 2.5 to 3.5 on a visible scale of 0 (clear) to 5 (opaque) and overall beauty. PA1 C. M.O.R. of at least 30,000 psi average. PA1 D. Ability to be heat treated to 925.degree.-950.degree. C. PA1 E. Structural integrity during heat treat PA1 F. Meltability and formability PA1 G. Chemical durability in an oral environment
The subject invention includes glass-ceramic materials which meet the above set forth criteria.